In industrial environments such as those containing weld smoke produced from the welding of aluminum, magnesium, steel, and similar metals, the clean up of such contaminated air prior to either reuse or discharge is a serious problem. Generally, prior to the enactment of more stringent environmental laws concerning clean up of air before discharge to the atmosphere, the problem was so great that such contaminants were usually merely discharged to the atmosphere. Now the problem has to be met, since the discharge of such contaminating exhausts is being more strictly controlled.
The two most conventional systems for separating contaminants from a gas stream include the use of either a stationary or movable filter media in the path of the gas stream (mechanical separation) or the use of an electrostatic filter. In the first system, a filter media material is utilized in either a fixed or movable system and as the air passes through the media, the pores or openings in the media allow the air to pass through while most of the contaminants are mechanically caught or trapped by the fibers or mesh of the filter material. In the case of weld smoke, however, the contaminants are so finely dispersed in the air stream that they pass right through the pores or openings in conventional media with the result that the percentage of contaminants removed is too low. Fiberglass is the most usual material for such media and if it is attempted to manufacture the fiberglass media with a tighter constructon, the pressure drop across the media becomes so great that the system is not efficient. In other words, the increased energy consumption of the system resulting from the larger size fan necessary to push the air through the tight fiberglass media is highly undesirable.
In the second method, while an electrostatic filter is initially very efficient and works well, the problem with utilizing it to separate contaminants from weld smoke is that a film of contaminants soon builds up on the plates of the electrostatic filter and provides an electrically insulating barrier. The electrostatic filter then becomes very inefficient. In an environment containing weld smoke such an insulating layer builds up after a couple of hours use. It is then necessary to wash the plates of the filter and allow them to dry before the electrostatic filter can again be used. Thus the electrostatic filter must be removed from service for a few minutes to an hour every two to three hours. As can easily be seen this is therefore completely unacceptable.